A prior art of a pneumatic regulator for supplying a constant pressure to a pneumatic apparatus is disclosed in Japanese Patent Publication No. 7-50418, for example.
This pneumatic regulator is an apparatus which is constructed such that, at the time of regulating a discharge pressure discharged from a main valve, the pneumatic regulator detects the discharge pressure, compares the detected discharge pressure with a predetermined pressure, and adjusts a pulse-width of a pneumatic pressure pulse supplied to a diaphragm chamber (pilot chamber) from a solenoid valve corresponding to the comparison result, then, a pilot pressure is increased or decreased corresponding to the pulse-width of the pneumatic pulse supplied from the solenoid valve, and an air supply valve body connected to the diaphragm chamber is opened or closed so as to regulate the discharge pressure discharged from the main valve to the predetermined pressure.
In the above-mentioned pneumatic regulator which constitutes the prior art, a drive circuit of the solenoid valve is constructed such that a so-called pulse-width modulation signal in which duty factor varies at a constant frequency is supplied to an electromagnetic coil (exciting coil) of the solenoid valve.
Accordingly, in case the discharge pressure from the main valve is set to a constant value, for example, a feedback control is performed to supply the pulse-width modulation signal to the electromagnetic coil in such a manner that at the time of starting the drive of the solenoid valve, the discharge pressure value is zero and hence, the duty factor of the pulse-width modulation signal is maximum, while the duty factor of the pulse-width modulation signal Is decreased as the discharge pressure value approaches a predetermined pressure value.
However, the inventors have found that in case the electromagnetic coil is driven by the pulse-width modulation signal, the electromagnetic coil is driven for a constant period and hence, in a condition where the discharge pressure value is close to the predetermined value (target value), an amount exceeding the predetermined pressure value (a so-called overshoot) becomes relatively large so that the stability in the vicinity of the predetermined pressure value is deteriorated.
The present invention is made in view of the above problems and it is an object of the present invention to provide a pneumatic regulator which can make the discharge pressure value smoothly approach the predetermined pressure value and can minimize the fluctuation of the discharge pressure value after it reaches the predetermine pressure value.